The polymers created in the synthesis of polyamides by polymerizing ε-caprolactam contain low molecular components that consist of caprolactam and their oligomers. In practice, these low molecular components are removed by extraction with hot water. From these extraction waters, one can recapture the caprolactam components, clean them and perhaps introduce them again into the polymerization. It is also possible to react the oligomers obtained in the extraction waters into caprolactam by adding splitting reagents and by then isolating, washing and reusing it.
Most known procedures have the disadvantage that sometimes, the reprocessing of the extraction water must take place in several steps before the entire extraction or the extracted components, especially ε-caprolactam can be used again for polymerization. The syntheses, which suggest separation, processing and recycling of caprolactam also have the disadvantage that the oligomers contained in the extraction waters are often not reprocessed, but must be decontaminated. Moreover, in the named syntheses for reusing extraction water, the use of a processing step for hydrolytic polymerization of the extraction water concentrate or a mixture of extraction water components and caprolactam is assumed.
Depending on the temperature, the raw polymer product obtained from the polymerization of polyamide-6 contains 8-15% by weight of caprolactam in equilibrium and its oligomers. These disrupt further processing and are therefore most often removed from the polymer matrix after granulation by extraction with hot water. The water used in the process, the extraction water, is evaporated in a multi-step distillation system (“recapturing system”) and the residual is put into the polymerization again as raw material.
However, in general there is a goal of keeping this amount of water as small as possible, because it requires energy to evaporate water.
In the known processes, polyamide-6 polymer is cut into cylinders or spheres in underwater granulators (UWG) or in underwater strand granulators (USG). Both types of granulators need cooling liquids in order to let the molten polymer solidify and to subsequently cool the polymer particles. This cooling liquid is normally water that is circulated in an almost closed loop.
In the course of the procedure, this water becomes enriched with caprolactam and oligomers from the polymer so that from time to time, clean water is added to the water that runs in the loop and it must be refreshed with such. Moreover, the evaporation and leakage losses must be adjusted.
Clean, most often desalinated water is added to the granulator systems. The water that is removed from the granulator system is either discarded or conveyed to the recapturing system, in order to recapture caprolactam and the oligomers. Utilization of the extraction water depends on the individual case, on the extraction content, and on the price of caprolactam and energy costs. In some cases it is more economical to discard caprolactam than to evaporate the water.
In the procedures mentioned above, technically, the continuous or also the discontinuous extraction of PA-6-chips with hot water has established itself. With this procedure, a monomer and oligomer content of <0.5% by weight are obtained in the PA-6-chip. Such low monomer and oligomer contents in polymers are required when the polyamide is to be used for spinning mill purposes.
For reasons of economy, the watery extracting solutions are processed in such a way that the valuable substances contained in them can be input as raw material into the polycaprolactam synthesis process. After simply concentrating the extraction water by evaporating the water, in addition to monomeric γ-caprolactam, the cyclical diners and additional oligomers also remain in the residual caprolactam.
In DE 2501348 B1, the concentration of the extraction water to more than 90% of the weight of the extraction amount with subsequent direct introduction into the polymerization step with and without adding fresh caprolactam is described. According to EP 0000397 B1, extraction water can also be recycled into the polymerization that was concentrated to a maximum of 60% of the extraction amount. In both cases, the extraction solutions—with or without the addition of fresh caprolactam—are adjusted in temperature prior to addition into the precondensation tube so that the cyclical diners of ε-caprolactam with high melting point remain in the solution under these conditions, so that no clogging of the pipes and the like occurs. But the splitting of the cyclical diners, which is necessary for subsequent insertion into the polymer chain can, however, not be sufficiently ensured in this way.
EP 0771834 A1 describes concentrating the extraction water with a subsequent partial ring-opening reaction of the oligomers into linear condensable compounds under reaction conditions of 230° C.-300° C. at defined pressures that are maintained up to 10 h. The thus treated extracts are subsequently polymerized together with fresh caprolactam in a reactor, whereby sometimes water concentrations of up to 10% by weight can be present. In U.S. Pat. No. 5,218,080 A, hydrolytic diner splitting of the concentrated extract is performed under pressures of 200-290° C. during a period of 2-6 h, whereby the thus obtained extract containing diners of approx. 1.3% by weight are added directly to the fresh lactam in quantities up to 10% by weight. Given the background of increasing capacity expansion of continuously operated hydrolytic caprolactam polymerization systems, the economy of these procedures and/or the amount of the residual dimer content in the extraction processed is thus in need of improvement.
Moreover, a process is known in which the extraction water after concentrating it to approx. 80% by weight, γ-caprolactam/oligomers without the addition of fresh caprolactam is polymerized in a second, separate polymerization line into PA-6 (Chemical Fibres International 47, 316 (1997)). The disadvantage of this synthesis is the high investment cost for a complete second polymerization line in which dimer reactivation takes place subject to polymerization conditions deviating from those of the fresh caprolactam polymerization process of the first line. The increased amount of water worsens the economics of this second line.
Other processes for the recovery of oligomers and cyclical diners that occur in the extraction water require separation of these components from the extraction water. U.S. Pat. No. 5,653,889 A describes a filtration technique for separating oligomers from the processing water of the PA-6 granulation. This filtration technique cannot be easily transferred to oligomer separation and processing of up to 15% by weight of watery extraction solution from the polymerization which also contains monomeric ε-caprolactam.
For the preparation of oligomers, a synthesis according to U.S. Pat. No. 4,107,160 A can be used, whereby—in addition to PA-6 solid substance waste—the oligomers are de-polymerized in the presence of a catalyst and overheated water vapor. After subsequent concentration, an approx. 50% by weight watery ε-caprolactam solution can be obtained, that is then, as per DE 4316408 A1, evaporated after a refinement step with permanganates and filtered with charcoal and evaporated; after fine distillation, the pure caprolactam that is obtained can be recycled into a PA-6 synthesis process. This costly procedure which yields a high quality of residual caprolactam, is accompanied by numerous procedural steps with correspondingly high energy consumption and materials such as permanganate and charcoal, and thus increased costs.
The alternatively possible discarding and decontamination of the oligomers that are isolated from the extraction water significantly reduces the raw material yield and thus does not represent an economical process, particularly for cases of rising system capacities.
Further, GB 1,297,263A mentions use of a catalyst for de-polymerization of the oligomers. As a possible catalyst, phosphoric acid is mentioned. What happens with the de-polymerization product is, however, not described there. Particularly, the mentioned British published patent specification does not mention a further addition of overheated water vapor into this splitting step.
In DE-A-43 21 683 and in U.S. Pat. No. 4,049,638, procedures for the synthesis of polycaprolactam are described that allow use of caprolactam with up to 15% water content in the polymerization. EP-A-0 745 631 reveals the re-use of watery extraction solutions by adding small quantities of a dicarbonic acid or polycarbonic acid, as otherwise the extract polymerizes slower than caprolactam.
As the extract also contains considerable amounts of cyclical oligomers which remain unchanged in the polymerization, several procedures for splitting these oligomers or transforming them into linear oligomers were proposed. The oligomers are usually split with phosphoric acid or by using high temperatures. Thus, U.S. Pat. No. 5,077,381 describes a procedure for splitting oligomers at temperatures of 220° to 290° C., preferably, subject to increased pressure.
Prior to returning the extraction solution into the polymerization, usually, the approximately 10% by weight extraction solution must first be processed, i.e. as a rule, it must be concentrated. Processing normally takes place by distilling the water. DE-A-25 01 348 describes the concentration process taking place in the absence of atmospheric oxygen, whereby prior to the concentrating to more than 70% of weight, fresh caprolactam is added to the extraction water, whereby the precipitation of oligomers is reduced.
In the application of the procedure for re-introducing extraction water that is mentioned above, there is, however, a severe disadvantage: The continual recirculation of the extraction water is subject to a significant increase in concentration of the oligomers and the thermodynamically stable cyclical dimers, not only in the reaction mixture, but also in the polymer, when, in the course of the continuous hydrolytic lactam polymerization, the splitting of oligomers is not successful, or the establishment of the chemical equilibrium is too slow. Moreover, the increase in oligomer concentration is particularly high when the reaction mixture—for example for the synthesis of polyamides with high molecular weight—has low water content.